Media access control (MAC) in a packet switched network is a mechanism for determining which of a plurality of nodes is allowed to send information over a shared medium. In a point-to-multipoint configuration it is common that the head end or central node in a shared link will control the utilisation of the link by multipoint nodes. This is achieved by polling the various multipoint nodes. The polling order and frequency is generally controlled according to traffic demand. In a time division multiplexed TDM link, time slots are allocated by the head end of a link in response to requests sent by the multipoint nodes when traffic is awaiting transmission at these node. The slots are allocated using permits that designate which time slot in the future traffic stream may be utilised for which service traffic. When more traffic is waiting than there are available time slots, the central node will issue queuing and prioritising requests to determine which service should be allocated channel capacity first, and which can wait.
Packet switched networks are increasingly utilised to transport a variety of traffic, some of which is traditionally circuit switched. An example of this is the transmission of voice over an internet protocol (IP) based network. While speech information may be packetised, certain transmission delays and delay variations must be respected if the speech service is to be acceptable to the end user. This is a general requirement of all circuit emulated traffic. Accordingly, with the above-described request based protocols, a multipoint node carrying circuit emulated traffic requires permits to be allocated without delay and with high priority. When more than one delay sensitive service utilises the shared medium the access control becomes more complex. This situation is made still more difficult when the delay sensitive services use different frame lengths and possibly also different bit rates so that intervals between the required high priority permits are always changing. The result is often excessive delays in one or more of the delay sensitive services leading to the end user experiencing an unacceptable quality of service.